To build the back details without departing from the concept, I gathered some elements of ornaments from other parts.

Textures

After finishing the high poly and low poly it was time to do the UV mapping to get the textures. I organized the UV layout into three parts to have better texture resolution: 2 x body and 1x shield, all at 2048.

With the mapping done, it was time to make the bakes. To help me in this process I used xNormal. For each piece I did three bakes: ambient occlusion, normal map and cavity map.

After the bakes were ready, I used Photoshop to join the pieces and make the diffuse map. To give the appearance of rusty metal, besides using the occlusion and cavity maps, I duplicated the cavity and inverted the colors and used it as Multiply to generate dirt in the corners, like rusted metal. For the painting, I left the concept parallel and picked colors to use as a color overlay.